The present invention relates to demineralizing whey.
Whey is a by-product of the milk and cheese industry, and it possesses great potential for development both concerning its lactose and concerning the serum proteins it contains.
Development is made difficult because of the presence of a large quantity of inorganic salts which firstly must be eliminated in order to make the final product suitable for consumption, and which secondly constitute factors that give rise to technical and economic constraints in the treatment of whey that are difficult to control.
Several methods are in existence for extracting from whey at least some of the inorganic salts it contains. Of these methods, mention can be made of ion exchange, electrodialysis, nanofiltration, . . . All of them have been operated on an industrial scale with greater or lesser success concerning the results obtained and above all concerning the cost of implementation, i.e. investment costs and running costs.
Attempts have also been made to associate them with one another in order to use them in series and take advantage of the maximum performance of each. It has found that the results obtained by one of them sometimes lead to a decrease in the performance of the following one, particularly since those techniques do not generally share the same know-how. For example, ion exchange and electrodialysis belong to two different technical fields which are not generally mastered by the same people.
The present invention stems from an observation concerning the behavior of whey with respect to membranes, whether for electrodialysis or belonging to apparatuses or units for nanofiltration. It has thus been discovered that the transfer of divalent ions (anions or cations) from whey occurs only with difficulty through such membranes, such that in order to obtain good results, economic conditions become prohibitive. It is thus possible at great expense to extract from whey some of the calcium or magnesium ions it contains; whatever the economic means implemented, it is very difficult to extract phosphate and citrate ions.
By means of the present invention, an optimum organization of various demineralization techniques is proposed in order to obtain highly demineralized whey at better cost and with minimum loss of the useable material while nevertheless maintaining the intrinsic quality thereof, in particular at microbiological level, by preserving it for example from lactose being degraded into lactic acid, proteins being degraded into non-protein nitrogen (NPN), . . . By means of this organization, the performance of separation apparatuses using membranes such as electrodialysis apparatuses or nanofiltration units is improved very substantially, thereby making it possible to reduce the surface areas of the membranes required, and thus making it possible to reduce running costs, with the additional consequence of obtaining low manufacturing costs for the whey demineralization treatment, regardless of whether it is treated in the raw or unconcentrated state or in the concentrated state.
To this end, the invention provides a method of treating whey for demineralization purposes, the method comprising a stage of separating out salts by transfer through electrodialysis or nanofiltration membranes, the method being remarkable in that, upstream from this separation stage, it comprises in succession at least one step of exchanging divalent cations for protons and at least one step of exchanging divalent anions for chloride ions.
This exchange of divalent cations for protons and of anions for chloride ions presents multiple advantages concerning both subsequent treatment implementing separation membranes and also the quality of the product itself. For example, exchanging cations for protons diminishes the influence of unstable proteins that are often associated with divalent cations and that tend to precipitate onto the surfaces of membranes, thereby requiring production to be stopped frequently for cleaning purposes. For example, since the step of exchanging divalent ions with chloride ions relates essentially to sulfates, the substance is relieved of the drawbacks of such sulfates which are firstly poorly ionized and therefore difficult to transfer through the membranes of electrodialysis apparatus, and which secondly pollute such membranes to a great extent. For example, the substance coming from those two ion exchange steps is a substance whose pH is relatively low, and the acid pH of this whey is a factor which enables citrate ions and phosphate ions to be transferred through nanofiltration membranes. Such transfer is practically non-existent in the presence of whey that has not been acidified. This performance is of considerable importance since, of all salts, it is known that phosphate ions are the most difficult to eliminate.
It should also be observed that having a substance with highly acidic pH at the outlet from these ion exchange steps is advantageous concerning control over microbiological quality. This substance lends itself in most advantageous manner to pasteurization at around 90xc2x0 C. to 100xc2x0 C. for a period of one or more minutes, which pasteurization serves to eliminate those germs that are the most difficult to destroy, i.e. sporulated germs, but without spoiling the proteins.
In a particularly preferred implementation of the invention, the step of exchanging divalent ions for protons comprises percolating the substance over a column of weak cationic resin, also known as carboxylic resin. One of the advantages of such a resin lies in the regeneration means that it requires, as explained below.
The extraction of divalent cations is improved by subsequently causing the substance that comes from said first carboxylic resin column to percolate over a strong cationic resin. Once 60% to 65% of the calcium and magnesium divalent cations have already been exchanged for protons by percolation over the carboxylic resin, the remainder of these divalent cations are exchanged for protons by the strong cationic resin. The strong cationic resin also makes it possible to find pH equilibrium that is preferably less than 3 by exchanging protons for monovalent ions of potassium and sodium.
Finally, the substance is passed over a strong anionic resin, preferably in a column which has mixed beds both of strong cationic resin and of the above strong anionic resin so as to exchange divalent anions for chloride anions. It is observed that this exchange applies essentially to sulfate anions.
This treatment leads to results that are entirely similar regardless of whether or not the whey is concentrated. Naturally, the quantities of resin to be implemented are proportional to the quantities of salts to be treated.
If the whey that has percolated over the resins is a concentrated whey, then, in accordance with the invention, the substance after these ion exchange steps is applied to the inlet of electrodialysis apparatus. By acting on a concentrated substance, the quantity of electric charge per unit volume is much higher than that present in non-concentrated whey, which acts in favor of using electrodialysis apparatus whose operation is then optimized from the point of view of electrical conductivity. Since the substance is also practically free from divalent ions, its operation is also optimized concerning the transfer of ions through cationic and anionic membranes. It has been observed firstly that the electrodialysis apparatus used in this way requires far less maintenance and upkeep, and secondly that the lifetime of the membranes is very significantly increased by factors of practically 1 to 2.
For non-concentrated xe2x80x9csoftxe2x80x9d whey, i.e. whey in which the concentration of dry matter is about one-third that of a concentrated xe2x80x9csoftxe2x80x9d whey, demineralization is preferably continued by using a nanofiltration unit. Since the ion exchange steps have eliminated most of the drawbacks existing in conventional nanofiltration of whey, this treatment stage possesses a first advantage of further demineralizing the whey since transferring chloride anions presents no difficulty, and since the acidity of the medium is high, the transfer of citrate and phosphate ions that have not been exchanged in the preceding steps is greatly improved. It possesses a second advantage which is that of extracting a very large quantity of water from the substance thus leading to a demineralized whey having a high concentration of valuable substances such as lactose and serum proteins. In this case also, it has been found that the performance of the nanofiltration operation is simply not comparable with that normally encountered when applying this treatment to raw whey.